In tires for trucks and buses and tires for commercial vehicles, in particular as the tread reinforcements thereof, steel cords having a 1.times.12 structure shown in FIG. 6, a 3+6 structure shown in FIG. 5 or a 3+9 structure shown in FIG. 4 are usually used.
In the 3+6 structure, three inner filaments (FC) are twisted in a direction, and six outer filaments (FB) are twisted in the reverse direction to the twisting direction of the three inner filaments (FC). That is, if the outer filaments (FB) are in S-direction, the inner filaments (FC) are in Z-direction, or vice versa.
In the 1.times.12 structure, however, penetration of its topping rubber into the cord is less, and a separation failure from the rubber is liable to occur.
In the 3+9 structure, also penetration of its topping rubber into the cord is less, and the adhesion of the cord with the rubber is poor.
In the 3+6 structure, if a strength required under heavy duty use is provided, the cord diameter inevitably increases, and the tire weight is also inevitably increased.
Further, in the 3+9 structure and 3+6 structure, when the steel cord is used as pull-resistant members, e.g. belt cords and the tire is subjected to a severe service condition, such as a high pressure, heavy load and high speed, the steel filaments are liable to be fretted, and the cord is finally broken.